1. Field of the Invention
The invention relates to an improved design for a heater blanket for use in decontamination of soil.
2. Description of the Prior Art
Assignee's U.S. Pat. No. 4,984,594 discloses an in-situ method for remediation and decontamination of surface and near-surface soils by evacuating the soil under a flexible sheet, which is impermeable to gases, and heating the soil surface with a relatively flat electric surface heater, which is permeable to gases. This patent discloses that the surface heater may be a mesh constructed from a conducting metal, such as stainless steel or resistance wire sold under the trademark, NICHROME, which can tolerate temperatures as high as 1000.degree. C. or more. The mesh has a total effective resistance of several ohms and good mechanical strength for durability when dragged over rough terrain. A 325 mesh of 304 stainless steel with wire 0.0014-inch in diameter is preferred. This patent discloses that the surface heater may also be constructed of metal wires which loop back and forth but do not form a mesh. For example, the metal wires could be stitched into a ceramic fiber cloth such as wire-reinforced NEXTEL 440 a trademark of Minnesota Mining & Manufacturing Co. of St. Paul, Minn. The surface heater could also consist of metal strips or ribbons or a metallic coating in a defined pattern. Still another possibility is to construct the surface heater from a thin metal foil or sheet with perforations to allow gas flow through the foil or sheet.
A preferred configuration for a permeable mat with good thermal insulation properties, according to the patent, is a layer of NEXTEL ceramic fiber combined with a layer of corrugated 304 stainless steel knitted wire mesh with about 1/4-inch crimp. The knitted wire mesh has extremely high gas permeability and can easily support a force of 15 pounds per square inch. Moreover, the thermal conductivity of such a knitted wire mesh is low. The permeable mat may be composed of multiple layers, including insulating layers, to reduce convection and radiation. Thus, the permeable mat may comprise a layer of NEXTEL ceramic fiber, then a layer of corrugated knitted wire mesh, then a layer of perforated aluminum foil for a radiation reflecting shield, then another layer of NEXTEL, another layer of knitted wire mesh, another aluminum foil shield, etc.
In Assignee's first soil remediation test, a 10'.times.10' electrical heating element was constructed from 325 mesh, 304 stainless steel, with 0.0014" diameter wires arranged in four 2.5'.times.10' strips in series. Total resistance of this heater at elevated temperature was about 1.3 ohms. The adjacent strips were separated by a 1" space and pegged to the ground to prevent movement. Strips were connected to each other by spot-welded stainless steel bus-bars. Copper power cables were bolted to the bus-bars at locations outside the heated zone.
After the test, inspection of the heater and insulation layers revealed that the heater had developed local hot spots and that a number of direct electrical shorts occurred through the NEXTEL cloth to the knitted stainless steel wire-mesh layer. Following the heater failure during this initial test, a new heater design, incorporating NICHROME wire elements woven into a NEXTEL cloth, and combined with a ceramic fiber insulating blanket, was installed in the test site. This alternative heater design used parallel 0.0095" diameter NICHROME wires, woven into a NEXTEL cloth. The NEXTEL cloth in turn served as the lower surface of a 1/2" insulating blanket of ceramic fiber. The upper surface of the insulation was covered with an "E"-Glass cloth. Thus, the heater and insulation under the flexible sheet was conveniently incorporated into a single blanket. Limitations on maximum size of wire that could be woven at the time by the supplier (3-M) resulted in a fragile heater element which, nevertheless, functioned well during the initial part of the test. By avoiding the metallic mesh, the bare wires were electrically isolated from each other with greater distances, and shorting was not experienced in the second heater.
The soil heater assembly (SHA) used during Assignee's tests at another site consisted of two, 5 ft by 10 ft heating sections. Each section contained three NICHRONIE heating elements encased in ceramic beads which were pinned to 4-inch thick ceramic fiber encased in NEXTEL cloth. The heating elements were run across the entire ten foot width of the SHA, spaced approximately three inches apart. The sections were then pinned to their respective 5 ft by 10 ft stainless steel frame. The heating sections were connected to a stainless steel support frame made out of structural angles and flat bars with pins that ran through the four-inch ceramic fiber insulation. Since the insulation was somewhat compressible and the pins not fixed, the heater elements could move vertically to accommodate surface irregularities of the soil. Both sections of the support frame were positioned side-by-side on the ground, bolted together, and covered by another four inches of fiber insulation. All of the insulation packages were surrounded by NEXTEL cloth for ease of handling and retaining their integrity.
Another of Assignee's invention disclosures disclosed an improved heating blanket for use in an in-situ method and apparatus for removing contaminants from surface and near-surface soil wherein a vacuum is imposed on the soil beneath an impermeable flexible sheet and the soil is then heated with an electric surface heating blanket positioned on the soil surface under the impermeable sheet. The heating blanket is permeable to vapors which emanate from the soil when heated and comprises a single piece heater construction. NICHROME 80 or Kanthal A.1 wires are substituted for NEXTEL threads in the weaving process which leaves the wires on one side of the cloth held in place by NEXTEL loops. This eliminates the need for a separate layer of NEXTEL insulating fabric over the heating elements.